In the process of drilling of wells into subsurface formations, it is common now to use "smart" motors at the end of the drillstring to adjust the rate and direction of drilling. Control of the motors is accomplished by means of signals from the surface. A number of known methods could be used for sending signals from the surface to a receiver at depth and vice-versa. This could be done by an acoustic signal carried by the mud or by the drillstring or it could be accomplished by an electromagnetic signal carried by the drillstring. These methods would be familiar to those versed in the art. However, these methods are difficult to use in continuing drilling operations because of the necessity of maintaining an adequate mud flow for drilling operations and of the noise associated with this and with the rotating drillstring. A common method of communicating the signals is by means of pressure pulses that alter the pressure of the drilling mud used in drilling operations. Prior art mud pulsing devices are generally classified in one of two categories. Either, the device generates positive pressure pulses or increases of pressure within the drill string over a defined basal level, or generates negative pressure pulses or decreases of the pressure for the drill string. U.S. Pat. No. 3,737,843, issued to Le Peuvedic, et al. is an example of a positive pulsing mud valve. A needle valve is mechanically coupled to a piston motor. The needle valve acts against a fixed seat. The piston motor in turn receives the continuous flow of control fluid. Information is transmitted to the surface in the form of rapid pressure variations ranging from 5 to 30 bars and succeeding one another at intervals of 1-30 seconds. Each pressure pulse is generated by reversing an electric current passing through a solenoid coil which is coupled to the needle valve.
Westlake, et al., (U.S. Pat. No. 4,780,620) shows a negative mud pulse system. A motor-driven valve is open in response to binary signals generated by a downhole sensor package. Upon opening the valve, portion of the mud flow is allowed to escape from the drill string to the annulus between the drill string and borehole.
Kotlyar (U.S. Pat. No. 4,703,461) discloses a device in which multistage mud pulsing is achieved by generating both positive and negative pulses within a drill string by means of a plurality of selectively operable bypass passages around a restriction to primary mud flow within a drill string or by venting to the outside of the drill string.
A major accompanying problem is that the signals get attenuated and dispersed as they propagate through the drilling mud. The attenuation and dispersion are unavoidable and are caused by various mechanisms, including viscous dissipation in the drilling mud as well as frictional energy loss at the borehole walls. The attenuation and dispersion of the signal becomes a particularly serious problem when underbalanced drilling mud is used to minimize reservoir damage. In normal drilling operations, or in drilling operations in geopressured formations where the risk of blowouts is high, the weight of the drilling mud is kept high enough so that the pressure of the mud exceeds hydrostatic pressure. In under-pressured reservoirs, use of heavy drilling mud could result in serious formation damage. Accordingly, drilling in such under-pressured reservoirs is carried out with underbalanced drilling muds that may contain nitrogen in the mud to reduce its density. The effect of the addition of nitrogen is to greatly increase the compressibility of the drilling mud: this reduces the bulk modulus and the velocity of propagation of the pulses in the drilling fluid. One result of an increased compressibility of the fluid is that a given pressure pulse at the source produces an increased flow pulse. In such a two-phase system consisting of a relatively incompressible liquid and a highly compressible gas, viscous dissipation greatly increases the attenuation and dispersion of mud pulse signals. For purposes of this application, any reference to a "compressible fluid" is intended to include a dissipative and attenuative fluid.
Another consequence of having a two-phase mixture of mud and a gas follows from the fact that the density and speed of propagation of sound in a gas (and a gas/liquid mixture) increases as the pressure is increased. When, as is typical in mud telemetry systems, the pressure pulses are comparable in magnitude to a "background"pressure, the trailing edge of a positive pulse may move faster than the leading edge. This greatly affects the shape of the pulse and complicates the process of pulse decoding.